Why would a landlord not have a lease agreement?

[Anthony Grandinetti]

I’m buying my first multi family deal. It’s a quadplex. 3 of the units are currently rented out. But the owner only has one lease. I’m wondering why she would do this, and what it means to me if I buy it. Should I request for her to establish lease agreements before I buy? How do I know what the rent really is? Could she be lying about the income? Please help me.